Optical Studies of Nitrogen to 130 GPa

Abstract

Nitrogen is observed to remain a molecular solid up to 130 GPa, contrary to recent theoretical predictions of metallization below 100 GPa. Raman scattering reveals three new phases at 20, 66, and 100 GPa, which are distinguished by branching of existing vibronic modes. One mode increases in frequency to a broad maximum at 66 GPa and then decreases, similar to the case of H 2 . In H2, such behavior was attributed to a weakening of the H-H bond. In N2, three other vibronic frequencies continue to increase, showing the situation to be more complicated than just a weakening of the N-N bond.PACS 78.30.Gt There has been a great deal of recent interest in insulator-metal transitions, particularly in diatomic molecules such as N 2 , H 2 , and I2. 1 Recent experimental and theoretical results for nitrogen indicate that N 2 might transform to a monatomic metallic state at pressures below 100 GPa (1 Mbar). The results of experimental studies by Nellis and co-workers 2 ' 3 of singleand double-shocked nitrogen appear to show a continuous dissociation of molecular nitrogen to a monatomic state starting at 30 GPa and 6000 K. Theoretical calculations by McMahan and LeSar 4 indicate that crystal structures composed of N 2 molecules may be less stable than a monatomic simple cubic structure at 0 K and 77 to 94 GPa. They suggested that structures observed in other group-V elements might have even lower cohesive energies than the simple cubic structure at high pressures. Martin 5 has subsequently made calculations which show that nitrogen would have a lower cohesive energy in the arsenic (A 7) rhombohedral structure at pressures around 100 GPa.Our work was intended to investigate the prediction that molecular nitrogen would transform to a monatomic structure, possibly metallic, at pressure accessible to the diamond-anvil cell. Preliminary experiments which we performed at pressure above 110 GPa showed a color change in nitrogen, but no dramatic transition to a metallic phase. Concurrently, Hemley, Mao, and Bell 6 made preliminary Raman measurements on nitrogen up to pressures of 160 GPa. They showed that the frequency of the v 2 vibron increases with pressure to a maximum at about 80 GPa and then drops, in a manner similar to the behavior of the vibrational mode of molecular hydrogen at 50 GPa. 7,8 These experimental developments prompted us to study the optical absorption spectrum of nitrogen and to investigate the N 2 vibron region carefully with high-resolution Raman spectroscopy in order to detect any subtle phase changes in molecular nitrogen at pressure.Our high-pressure experiments were made with "megabar" diamond-anvil cells. 9 We employed the techniques described by Goettel, Mao, and Bell, 10 and used beveled-diamond anvils for generating pressures above 100 GPa. The anvils used were standard brilliant-cut diamonds with sixteen facets, a central flat of 50 jxm, a 5° bevel, and a 300-/xm total culet diameter.The sample chamber was formed by preindenting a 250-/xm-thick 7-301 stainless-steel gasket to a thickness of 15 to...

First-principles equation of state calculations of warm dense nitrogen

“…At low T, nitrogen forms a molecular solid that undergoes a series of solid phase transitions with increasing pressure (see Fig. 1), which have been identified by a number of static compression experiments [10][11][12][13][14][15] . Around 50-70 GPa, density functional molecular dynamics (DFT-MD) simulations first predicted [16][17][18][19][20][21] the triple bond would destabilize to form various lower-energy, nonmolecular (possibly amorphous), polymeric phases composed of doubleor single-bonded atoms, such as cubic gauche 18 .…”

High-pressure amorphous nitrogen

“…There is evidence for further pressure-induced transformations of the high-pressure molecular ζ phase but the products are believed to be closely related. 15,25 Dotted line is extrapolation of the ζ-η transformation boundary.…”

THERMODYNAMIC QUANTITIES AT HIGH PRESSURES IN THE i AND θ PHASES OF SOLID NITROGEN DEDUCED BY RAMAN FREQUENCY SHIFTS FOR THE INTERNAL MODES IN LITERATURE

“…This is due to different structures of both phases. As also indicated in previous Raman studies, 9,23,24 some of the intramolecular vibrations soften when the pressure increases, which can be related to the weakening of intramolecular bonding or to the increase in vibrational coupling. 25 It has been indicated that there is a more pronounced Raman and IR softening of the vibron bands of θ nitrogen as compared to the i nitrogen.…”

“…Also, by Raman spectroscopy the P -T phase diagrams of solid N 2 have been determined. 8,9,[14][15][16][17] Far-infrared study on the α-N 2 , 18 X-ray measurements for the δ phase, 6,14 the Raman scattering and X-ray diffraction 10,14,16 and also Brillouin scattering 15 studies on the ε-N 2 have been reported in the literature. Another phase η-N 2 has been observed by the Raman 9 and X-ray diffraction 19,20 measurements.…”